Length Converter.
Convert between metric, imperial, nautical, historical, and astronomical length units instantly. Covers cm, inches, feet, meters, miles, kilometers, light years, and more.
Precision
How many significant digits to display in results.
Number Format
Always show exponential form (e.g. 3.94e+1). Enabled automatically for very large or very small values.
Unit Reference
Metric — nm to km, powers of 10. 1 m = 1/299 792 458 light-second exactly.
Imperial — 1 in = 25.4 mm, 1 ft = 12 in, 1 yd = 3 ft, 1 mi = 5 280 ft.
Nautical — 1 nmi = 1 852 m (one arc-minute of latitude). 1 fathom = 6 ft.
Historical — 1 chain = 22 yd, 1 furlong = 10 chains, 1 league = 3 miles.
Astronomical — 1 AU ≈ 149.6 Mkm · 1 ly ≈ 9.46 × 10¹⁵ m · 1 pc ≈ 3.26 ly.
Unit Reference
Metric System (SI)
The International System of Units defines the metre as the base length unit. All metric lengths are powers of ten: 1 km = 1 000 m, 1 cm = 0.01 m, 1 mm = 0.001 m. The metre was redefined in 1983 as exactly 1/299 792 458 of the distance light travels in vacuum in one second.
Imperial & US Customary
The international yard was standardised in 1959 to exactly 0.9144 m, fixing all derived units: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 mile = 1 609.344 m — all exact. Imperial units remain in daily use in the USA and partially in the UK.
Nautical Units
The nautical mile (1 852 m exactly since 1929) was originally one arc-minute of latitude on the Earth's surface. It is the standard unit in maritime and aviation navigation. A fathom equals 6 feet (1.8288 m) and is used to measure water depth.
Astronomical Distances
The AU is the mean Earth–Sun distance: 149 597 870 700 m exactly. A light year is the distance light travels in one Julian year: ~9.461 × 10¹⁵ m. A parsec is the distance at which 1 AU subtends one arc-second: ~3.086 × 10¹⁶ m ≈ 3.26 light years.
How length conversion works
All conversions go through a common base — the metre. Multiplying by the source unit's metres-per-unit factor converts to metres; dividing by the target unit's factor converts from metres to the result. For example, 5 feet to centimetres: 5 × 0.3048 ÷ 0.01 = 152.4 cm.
Centimetres to inches
1 inch = 25.4 mm exactly, so 1 cm = 1/2.54 inches ≈ 0.3937 in. To convert cm to inches, divide by 2.54. Common reference: 30 cm ≈ 11.81 in, a standard ruler length.
Feet and metres
1 foot = 0.3048 m, equivalently 1 m ≈ 3.281 ft. Height is stated in feet and inches in the USA; most other countries use centimetres. A person 6 ft tall is 182.88 cm.
Kilometres to miles
1 mile = 1 609.344 m = 1.609344 km. To convert km → miles, divide by 1.609344. Quick estimate: multiply km by 0.6 (exact factor ≈ 0.6214). 100 km ≈ 62.14 miles.
Yards to metres
1 yard = 0.9144 m exactly. A football field in the USA is 100 yards = 91.44 m. Fabric and carpet are often sold in yards; dividing yards by 1.094 gives approximate metres.
Frequently Asked Questions
- How many centimeters are in an inch?
- Exactly 2.54 centimeters equal one inch (by international definition since 1959). To convert cm to inches, divide by 2.54. To convert inches to cm, multiply by 2.54.
- How do I convert kilometers to miles?
- 1 mile = 1.609344 km exactly. To convert kilometers to miles, divide by 1.609344. Quick estimate: multiply km by 0.6214. For example, 100 km ≈ 62.14 miles.
- What is a nautical mile?
- A nautical mile (nmi) equals exactly 1,852 metres — originally defined as one arc-minute of latitude on Earth's surface. It is the standard distance unit in maritime and aviation navigation worldwide.
- How accurate are the astronomical unit conversions?
- Very accurate. The AU is defined as exactly 149,597,870,700 m. The light year is based on the Julian year (365.25 days) giving ~9.461 × 10¹⁵ m. The parsec is ~3.086 × 10¹⁶ m — all values meet scientific publication precision.
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